In the disclosure of the present invention reference is mostly made to drug delivery devices used e.g. in the treatment of diabetes by delivery of insulin, however, this is only an exemplary use of the present invention.
Drug delivery devices allowing for multiple dosing of a required dosage of a liquid medicinal product, such as liquid drugs, and further providing administration of the liquid to a patient, are as such well-known in the art. Generally, such devices have substantially the same purpose as that of an ordinary syringe. Drug delivery devices of this kind have to meet a number of user specific requirements. For instance in case of those with diabetes, many users will be physically infirm and may also have impaired vision. Therefore, these devices need to be robust in construction, yet easy to use, both in terms of the manipulation of the parts and understanding by a user of its operation. Further, the dose setting must be easy and unambiguous and where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose. In order to meet these requirements, the number of parts and steps required to assemble the device and an overall number of material types the device is made from have to be kept to a minimum.
Typically, the liquid drug to be administered is provided in a cartridge that has a moveable piston or bung mechanically interacting with a piston rod of an expelling mechanism of the drug delivery device. By applying thrust to the piston in distal direction, a predefined amount of the liquid drug is expelled from the cartridge. Due to inevitable manufacturing tolerances there may for instance persist axial clearance between a cartridge's piston and the piston rod. Typically, prior to a primary use of the device, an end-user has to conduct a so-called priming of the expelling mechanism in order to ensure, that already with an initial dose setting and a first subsequent dose dispensing step, an accurate amount of the liquid drug is dispensed in a predefined way. An initial dose setting and expelling of a minor dose may in certain situations also be required for removing any air present in the cartridge and/or a connected needle and for performing a flow check.
Document WO 99/38554 A1 discloses several embodiments of injection devices each suitable for forming a disposable device wherein a liquid drug cartridge is inserted into the device during assembly in a production line.
State of the art pen-type drug delivery devices that incorporate a dose setting feature often include a so-called end-of-content limiter to prevent a user from selecting a size of a dose which exceeds the amount of liquid drug remaining in a cartridge of the device. References WO 01/19434 A1, WO 2006/128794 A2, WO 2010/149209 A1 and WO 2013/156224 A1 include disclosure of such end-of-content limiters.
In the production line, during final assembly operations of the devices, at least a part of the priming is typically carried out using the dose setting and expelling mechanism so that users will experience virtually consistent requirement for a priming operation across individual pen samples irrespective of the initial gap between the piston rod and the piston which emanates from tolerance variations. Reference WO 2009/095332 A1 discloses devices wherein a distance between the distal end of a piston rod means and the plunger is minimized or reduced to zero. WO 2011/113868 A1 provides a further disclosure of related methods and devices.